"An improved method for preparing gravimetric standard gas mixtures of helium-4 in nitrogen has been developed by the U.S. Bureau of Mines. A single-pan 12-kg analytical balance that has a sensitivity of 3 mg is used to prepare the mixture. The balance will accommodate a cylinder having a volume of approximately 4 l that weighs approximately 5 kg. The method has been improved to a precision of +/-0.04 pct by performing four successive dilutions, by improved handling of gases in the manifold, and by using an improved analytical method to select high-purity gases." - NIOSHTIC-2 NIOSHTIC no. 10008167

National Institute for Occupational Safety and Health